Pulsar and companions will put general relativity to the test

Astronomers have discovered the first pulsar with two stars circling it. By watching the three objects orbit one another, observers will soon be able to perform the best-ever test of the "strong equivalence principle", which is a key prediction of Albert Einstein's general theory of relativity.

Like the Newtonian theory of gravity that came before it, Einstein's general theory of relativity says that gravity does not discriminate: it accelerates all objects equally, no matter what their size, shape or composition. Apollo 15 astronaut Dave Scott demonstrated this so-called equivalence principle on the Moon in 1971 by dropping a hammer and a falcon's feather, which hit the lunar surface simultaneously.

The strong equivalence principle of general relativity goes further, saying that gravity should accelerate all objects in the same way even if the objects hold themselves together with their own gravity. In other words, the gravitational self-energy that binds a planet or star together should have no effect on how it is accelerated. This is unlike theories that seek to topple general relatively and predict a small deviation related to gravitational self-energy called the Nordtvedt effect.

Three-body test

The most exacting test of the strong equivalence principle performed so far involves tracking the Earth and the Moon. As they orbit the Sun, both are continually falling through the solar gravitational field. Einstein's theory says that the Earth and the Moon should behave the same, even though the Earth has greater self-gravity. Precise laser-ranging measurements of the distance between the two bodies back this up by revealing no evidence of the Nordtvedt effect.

"The problem with tests of the strong equivalence principle here in the solar system is that none of the objects is strongly self-gravitating," says Scott Ransom of the National Radio Astronomy Observatory in Charlottesville, Virginia. In contrast, a pulsar is ideal. It arises when a massive star explodes and collapses; it is typically just 20 km across but about 50% more massive than the Sun, so its gravity strongly binds it together.

Now, Ransom and colleagues have discovered a pulsar named PSR J0337+1715 that will put Einstein to the test, thanks to the two stars that circle it. All pulsars spin fast, but this one, located 4200 light-years away in the constellation Taurus, spins especially quickly. It is a millisecond pulsar and each second it spins 365.953363096 times. Knowing its period to this incredible precision makes the pulsar an outstanding clock that astronomers can exploit.

Extraordinary and very rare

By recording when the pulsar's pulses reach Earth, Ransom's team discovered small delays caused by the gravitational tugs of two companion stars. Many millisecond pulsars have one stellar companion, which has dumped material onto the pulsar and spun it up to high speed. But astronomers have never before found a pulsar with two stellar companions. "It's really in a pretty extraordinary and very rare system," says Ransom.

Both companions are white dwarfs, which have weaker self-gravity than the pulsar. Both are larger than the Earth but less massive than the Sun. One white dwarf is much closer to the pulsar than Mercury is to the Sun and orbits it every 1.629401788 days. The other white dwarf is about as far out as the Earth is from the Sun, circling the pulsar and the inner white dwarf every 327.257541 days.

If I was going to hope, I would hope that we show that [the strong equivalence principle] is wrongScott Ransom, National Radio Astronomy Observatory

The pulsar and the inner white dwarf can be thought of as Scott's hammer and the feather: both are falling through the gravitational field of the outer white dwarf. Although the pulsar has much greater self-gravity, Einstein says both it and the inner white dwarf should respond in exactly the same way.

By carefully monitoring delays in the pulsar's pulses, Ransom and colleagues are currently tracking the exact positions of all three objects. "Very, very soon we are going to be able to make tests of the strong equivalence principle that are orders of magnitude better than anyone's been able to do before," says Ransom, who expects a verdict within a year. "If I was going to hope, I would hope that we show that it's wrong. That would be a really great thing to move science forward."

Not the final word in gravity

So far, Einstein's theory has passed every test. "But general relativity is probably not the final word in gravity, since it doesn't mesh well with quantum mechanics," Ransom says. "So eventually, in some deep dark corner of parameter space, it's probably going to fail. This could be it; we just don't know yet."

Clifford Will, a physicist at the University of Florida in Gainesville and author of the book Was Einstein Right?, calls the new pulsar the greatest test the strong equivalence principle has ever faced. "I hope Einstein prevails," he says. "General relativity is so unbelievably simple by comparison [with alternative theories] that to me it just has the feeling that it has to be right, down to the quantum level." Einstein formulated general relativity in 1915, and Will says a confirmation in 2015 "would be a great 100th birthday present for Einstein's theory".

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Logic and General Relativity

Logic and General Relativity

Dennis Sciama said in the preface of his “the Physical Foundations of the General Relativity” that logical incompleteness of Newton’s laws of motion leads us to the Einstein’s General Relativity. Therefore some global and chronic problems in learning Newtonian mechanics have kept me engrossed for about 35 years. I think that we have not understood that logical incompleteness completely. Moreover, we do not pay sufficient attention to that incompleteness. Consequently, teachers’ negligence of logical aspects makes adverse effects on the learning and liking for physics. This is a major reason, I think, why physicists / teachers had to celebrate the year 2005 as the Einstein Year for the popularization of physics.

Also I think that due attention to logic was not given while organizing various events of 2005. So, I hope that more attention will be paid next year in the Centenary of General Relativity – because some astrophysicists have a feeling that exo-planets are questioning the planetary theories.

Sounds like good science to me. But I'm not clear why people say Einstein's general theory of relativity "accelerates all objects equally, no matter what their size, shape or composition". Einstein said a curvature of light can only occur when the speed of light varies with position. Then you can think of pair production and the wave nature of matter, and simplify an electron to light going round a square path. The horizontals curve down, so the electron falls down. But this doesn't work for the "frozen star" black hole where the coordinate speed of light is zero. And the frozen-star black hole is legit according to one interpretation of GR. Kevin Brown refers to this in the formation and growth of black holes. He doesn't like it, but he does refer to GR interpretations.

Surely from the previous experiments done to measure the Nordtvedt effect would not be measurable as they were done within a closed system (relativistically). But making the measurements of the stars around the pulsar would be making measurements outside of a closed system therefore would yield evidence of the Nordtvedt effect.

Relativity on track or physics at the precipice?

Should observations as described in this article not be explainable with GR do we really expect GR to Be declared obsolete? No, not at all everything will remain at the status quo and physics will just have another unsolved problem.

Should it be accepted that GR fails, it would be narrow minded to say the equivalence principle fails. It coud equally be that SR has another explanation not based on the Lorentz invariance, or both. In that case all physics fails, an unthinkable outcome, but the right step to reformulate QM that includes the gravitational phenomenom.

alv: the important point about the equivalence principle is that it applies to an "infinitesimal" region. It applies to a region of zero size. Which means it doesn't apply at all. A real gravitational field can be likened to accelerating through space, but it just isn't the same. It just isn't equivalent. Have a read of P M Brown's essay at arxiv.org…0204044 for more. Note this 1971 quote by Synge:

"...the Principle of Equivalence performed the essential office of midwife at the birth of general relativity, but, as Einstein remarked, the infant would never have gone beyond its long clothes had it not been for Minkowski’s concept [of space-time geometry]. I suggest that the midwife be buried with appropriate honours..."

The upshot is that when people say they're testing the principle of equivalence, they aren't testing general relativity. They're testing a misunderstanding of general relativity.

Equivalence Pricinciple and Nordtvedt Effect

Einstein identified the existence of gravity with the inertial motion of the accelerating bodies: bodies in free fall. To render this system in acceleration, inetial there has to be an equal and opposite force called gravity. However, this works only, when the 'inertial acceleration' of a body can be identified with its gravitational acceleration. This is the Equivalence Principle(EP), where the inertial mass of a body has to be equal to its gravitational mass whatever its size, form or composotion. In the Nordtvedt Effect the additional presence of the gravitational self-energy Eg should indeed disturb this equality of tyhe two types of masses and violate the EP. For the Sun the ratio of Eg/m(its inertial mass)is 3.6 10^-6, for the Earth: 4.6 10^-10 and for the Moon: 0.2 10^-10.

Should a neutron fall faster than a proton?

Free fall:If you drop a proton and a neutron in a gravitational field, they both fall, but the proton has a charge and accelerating charges radiate energy, so that leaves less kinetic energy for the proton and by this reasoning, it should fall more slowly than a charge-free object.

The question: Is the acceleration virtual or not? In the co-ordinate system of the particles, the acceleration is not felt, but the observer sees the particles to accelerate in his co-ordinate system. So, will the free falling protons radiate?

Same acceleration for neutron and proton

The gravitational self-energy creates a diffrence between the inertial and gravitational mass of a body. As this difference has to be negligeable for a neutron and a proton, they should have the same gravitational acceleration even when a proton radiates bremssralung.

Free fall:If you drop a proton and a neutron in a gravitational field, they both fall, but the proton has a charge and accelerating charges radiate energy, so that leaves less kinetic energy for the proton and by this reasoning, it should fall more slowly than a charge-free object.

The question: Is the acceleration virtual or not? In the co-ordinate system of the particles, the acceleration is not felt, but the observer sees the particles to accelerate in his co-ordinate system. So, will the free falling protons radiate?

SomeOneElse: The principle of equivalence relates accelerating through gravity-free space to "accelerating" when you're standing on the ground, not when you're falling. A falling charged particle doesn't radiate, and nor does it radiate if it's just sitting on the ground. It only radiates when you stop it falling.

Besides that, the neutron has its magnetic moment, which betrays the existence of charge. There's just no net charge, just as a hydrogen atom has no net charge.

Principle of relativity

In free fall, the gravitaional referencial becomes inertial and due to the principle of relativity, that of the observer goes over from the intertial to the non-inertial with the gravitational acceleration operating in the opposite direction.

SomeOneElse: The principle of equivalence relates accelerating through gravity-free space to "accelerating" when you're standing on the ground, not when you're falling. A falling charged particle doesn't radiate, and nor does it radiate if it's just sitting on the ground. It only radiates when you stop it falling.

Besides that, the neutron has its magnetic moment, which betrays the existence of charge. There's just no net charge, just as a hydrogen atom has no net charge.

GR failure ?

Such notions fall on the laughter of the gods ! Tested from the scale of the solar system to galactic clusters, it has passed Every test & has Never failed, & IMO never will fail, as long as it's applied to matter. Indeed, those expecting it to fail draw their suspicions from string theory & its suggestion of a `5th force' due to SUSY particles. However, SUSY has `tanked' at the LHC, & its supporters are hoping for an indirect hint of it from this 3-body test.Soon the AEGIS group at CERN will test the free fall of anti-Hydrogen atoms. When GR was created, anti-matter was unknown & remained so for another 17 yrs. SN1987A suggested that neutrinos & anti-neutrinos `fall' at the same rate, but it was a singular measurement of something quite ephemeral & nowhere near as robust as the A-Hydrogen test, which can be repeatedly checked.

Hydrogen and Anti-hydrogen

Normally the inertial and gravitational masses of a body should be same due to the Equivalence Principle. However,as said before, the gravitational self-energy Eg of the body should disturb this equality. As the Eg depends on the inertial mass of the body, to see its effect, one has to deal with massive bodies with very different masses. However, this is not the case for hydrogen and anti-hydrogen. Moreover, both hydrogen and antihydrogen should behave the same way under gravity.

Such notions fall on the laughter of the gods ! Tested from the scale of the solar system to galactic clusters, it has passed Every test & has Never failed, & IMO never will fail, as long as it's applied to matter. Indeed, those expecting it to fail draw their suspicions from string theory & its suggestion of a `5th force' due to SUSY particles. However, SUSY has `tanked' at the LHC, & its supporters are hoping for an indirect hint of it from this 3-body test.Soon the AEGIS group at CERN will test the free fall of anti-Hydrogen atoms. When GR was created, anti-matter was unknown & remained so for another 17 yrs. SN1987A suggested that neutrinos & anti-neutrinos `fall' at the same rate, but it was a singular measurement of something quite ephemeral & nowhere near as robust as the A-Hydrogen test, which can be repeatedly checked.

Is the question of the existence or non-existence of this "Nordtvedt effect" equivalent to the question of whether the strength of the "gravitational field" of a mass grows linearly with the number of neutrons in the mass, or non-linearly?

Such notions fall on the laughter of the gods ! Tested from the scale of the solar system to galactic clusters, it has passed Every test & has Never failed, & IMO never will fail, as long as it's applied to matter. .

And that is the problem, the closed paradigmyou exhibit, which echoes that of the majority of the science community, does not allow for progress. All major progress of mankind was always associated with a paradigm shift. Maybe the time has come, and with observations as described in this article the concrete of the Einsteinian paradigm will either continue to harden or it may start weathering on the edges.

Dark energy

Sounds like good science to me. But I'm not clear why people say Einstein's general theory of relativity "accelerates all objects equally, no matter what their size, shape or composition". Einstein said a curvature of light can only occur when the speed of light varies with position. Then you can think of pair production and the wave nature of matter, and simplify an electron to light going round a square path. The horizontals curve down, so the electron falls down. But this doesn't work for the "frozen star" black hole where the coordinate speed of light is zero. And the frozen-star black hole is legit according to one interpretation of GR. Kevin Brown refers to this in the formation and growth of black holes. He doesn't like it, but he does refer to GR interpretations.

Is dark energy possibly look at like gravity in terms of : "accelerates all objects equally, no matter what their size, shape or composition"? If Yes, can we measure it at this relativistic test also??

Special Realtivity needs testing too!

Special Relativity has been far less tested than Quantum Mechanics. Even Einstein's own suggested platform-and-traincar experiment, which can now be done in table-top form, has not been done.

Time dilation and length contraction are the two central consequences Special Relativity. Forget testing the accuracy of the Length Contraction equation, it has not even been tested that length contraction happens. To put it another way, length contraction has never been observed, and over a hundred years have passed since it was theorized.

General Relativity's failure to explain the observed universe is well known. The evidence against General Relativity is not just in the behavior of galaxies but Earth-side too in the behavior of various launched space-crafts. Dark matter was created to explain failures of General Relativity .

Billions of dollars are being allocated and are being spent to search for Dark Matter. If Special Relativity fails an experiment then no Dark Matter or other such creation can save it. And General Relativity is built on the assumption that Special Relativity is true. It might be wise to spend a tiny amount to at least confirm Special Relativity.

It is good to see that individuals are now committing personal money to help bring about experimental testing of Special Relativity. The owner of below site has committed $7 million.